1. Field of the Invention
The present invention relates to a method for cleaning semiconductor surfaces having uneven surface topography and more particularly relates to a method for ion-beam etching of semiconductor surfaces having uneven surface topography so as to clean the surfaces and provide a suitable surface for subsequent operations on the semiconductor surface.
2. Description of the Prior Art
Ion etching, which includes the processes of ion beam milling and RF sputter etching, is the process by which a substrate surface is slowly eroded by bombardment with a stream of high-energy ions. The erosion process is one of momentum transfer between the impinging ions and the atoms of the substrate, by which the substrate atoms receive sufficient momentum to be carried away from the substrate. This type of sputter erosion is well known for the purpose of etching or ion milling the surface of a substrate. For example, U.S. Pat. No. 3,528,387 to Hamilton describes a method for ion cleaning of a substrate and subsequent vapor deposition. The ions are emitted into a vacuum chamber through a small port generally aligned toward the substrate. The ions are used to bombard the substrate to clean its surface prior to deposition of a material, such as a metal film, onto the substrate. It is also disclosed that the ions may bombard the substrate during or after the deposition step to improve adherence of the deposited metal film on the substrate.
While ion etching, particularly ion beam milling, has been found to be very useful in cleaning the surface of a substantially planar substrate surface prior to effecting blanket deposition of a coating, such as a metal or insulator coating, there have been problems associated with ion etching when used to clean a substrate surface having topological irregularities in the surface. This problem is usually associated with the fact that the ions leaving the surface of the substrate tend to interact with ions impinging on the surface. This causes redeposition of the ions removed by the ion etching step and interferes with the milling operation. Various designs of catcher assemblies have been proposed to "catch" the substrate ions leaving the surface and prevent their redeposition. One type of catcher assembly is disclosed in an article of C. M. Melliar-Smith, "J. Vac. Sci. Technol., Vol. 13, No. 5, Sept./Oct. 1976, pp. 1008-1021. While catcher assemblies have been helpful in alleviating the problem of redeposition, they have not been wholly successful and it is desirable to provide a method for ion etching whereby the surface of substrates having topological irregularities can be cleaned effectively prior to performing other operations on the substrate surface.
It has been found that a substrate having an irregular surface, such as a channeled or stepped surface, can be effectively cleaned by ion milling if the substrate surface is oriented in a particular manner while it is exposed within the solid angle of an ion beam. The method is particularly adapted for cleaning via channels which are used for interconnecting layers of metal separated by an insulating layer.